Galaxies Coming of Age in Cosmic Blobs
A deep study of 29 gigantic blobs of hydrogen gas has been carried out with NASA's Chandra X-ray Observatory to identify the source of immense energy required to illuminate these structures. These mysterious blobs - called "Lyman-alpha blobs" by astronomers because of the light they emit - are several hundred thousand light years across and are seen when the Universe is only about two billion years old, or about 15% of its current age.
The composite image on the left shows one of the largest blobs observed in this study. Glowing hydrogen gas in the blob is shown by a Lyman-alpha optical image (colored yellow) from the National Astronomy Observatory of Japan's Subaru telescope. A galaxy located in the blob is visible in a broadband optical image (white) from the Hubble Space Telescope and an infrared image from the Spitzer Space Telescope (red). Finally, the Chandra X-ray Observatory image in blue shows evidence for a growing supermassive black hole in the center of the galaxy. Radiation and outflows from this active black hole are powerful enough to light up and heat the gas in the blob. Radiation and winds from rapid star formation occurring in the galaxy is believed to have similar effects. Clear evidence for four other active black holes in blobs is also seen.
The artist's representation on the right shows what one of the galaxies inside a blob might look like if viewed at a relatively close distance. A two-sided outflow powered by the supermassive black hole buried inside the middle of the galaxy is shown in bright yellow, above and below the spiral arms of the galaxy. This outflow illuminates and heats gas surrounding the galaxy. Radiation from regions close to the black hole will also play a significant role in lighting up and heating the blob. Stars are forming at a rapid rate in this galaxy, and young stars are being destroyed in supernova explosions. The three bright stars above the central bulge of the galaxy are examples of such supernovas (a companion illustration shows the effects of such explosions).
These new results show how blobs fit into the cosmic story of how galaxies and black holes evolve. Galaxies are believed to form when gas flows inwards under the pull of gravity and cools by emitting radiation. This process should stop when the gas is heated by radiation and outflows from galaxies and their black holes. Blobs could be a sign of this first stage, or of the second.
Based on the new data and theoretical arguments, Geach and his colleagues show that heating of gas by growing supermassive black holes and bursts of star formation, rather than cooling of gas, most likely powers the blobs. The implication is that blobs represent a stage when the galaxies and black holes are just starting to switch off their rapid growth because of these heating processes. This is a crucial stage of the evolution of galaxies and black holes -- known as "feedback" -- and one that astronomers have long been trying to understand.